Organic photovoltaic devices comprising fullerenes and derivatives thereof

ABSTRACT

Photovoltaic cells comprising an active layer comprising, as p-type material, conjugated polymers such as polythiophene and regioregular polythiophene, and as n-type material at least one fullerene derivative. The fullerene derivative can be C60, C70, or C84. The fullerene also can be functionalized with indene groups. Improved efficiency can be achieved.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/743,587, filed May 2, 2007, which claims priority to U.S. provisionalapplication Ser. No. 60/812,961 filed Jun. 13, 2006 to Laird et al.,which is hereby incorporated by reference in their entirety.

BACKGROUND

A need exists to provide better materials and processes for organicphotovoltaic (OPV) devices. This is driven in part by ongoing high fuelprices and unstable fuel supply. OPV devices can provide improvementsover older silicon devices. See for example Perlin, John “The SiliconSolar Cell Turns 50” NREL 2004; see also, Dennler et al., “FlexibleConjugated Polymer-Based Plastic Solar Cells: From Basics toApplications,” Proceedings of the IEEE, vol. 93, no. 8, August 2005,1429-1439. Global climate change is also a motivating factor. While itis known that conducting polymers, or conjugated polymers, including forexample polythiophenes can be combined with C60 fullerene to provideuseful active materials in OPV devices, a need yet remains to improvedevice efficiency and other important PV parameters. In particular,regioregular polythiophenes are of particular importance because oftheir nanoscale morphology which can be applied to novel morphologiesfor solar cell applications.

SUMMARY

Provided herein are, among other things, compositions, devices, methodsof making, and methods of using.

For example, provided herein is a composition comprising a mixturecomprising: (i) at least one p-type material, (ii) at least one n-typematerial, wherein the n-type material comprises a fullerene derivativerepresented by:F*-(R)_(n)and solvates, salts, and mixtures thereof,wherein n is at least one,

F* comprises a fullerene having a surface which comprises six-memberedand five-membered rings; and

R comprises at least one optionally substituted, unsaturated orsaturated, carbocyclic or heterocyclic first ring, wherein the firstring directly bonds to the fullerene.

Another embodiment provides a composition comprising a mixturecomprising: (i) at least one p-type material, (ii) at least one n-typematerial, wherein the n-type material comprises at least one fullerenederivative comprising at least one [6,6] fullerene bonding site whereinboth carbon atoms of the [6,6] bonding site are covalently bonded to agroup R.

Another embodiment provides a composition comprising a mixturecomprising: (i) at least one p-type material, (ii) at least one n-typematerial, wherein the n-type material comprises a fullerene derivativecomprising at least one fullerene covalently bonded by [4+2]cycloaddition to at least one derivative moiety.

Another embodiment provides a photovoltaic device comprising at leastone anode, at least one cathode, and at least one active layer, whereinthe active layer comprises a composition comprising a mixturecomprising: (i) at least one p-type material, (ii) at least one n-typematerial, wherein the n-type material comprises a fullerene derivativerepresented by:F*-(R)_(n)and solvates, salts, and mixtures thereof,wherein n is at least one,

F* comprises a fullerene having a surface which comprises six-memberedand five-membered rings; and

R comprises at least one optionally substituted, unsaturated orsaturated, carbocyclic or heterocyclic first ring, wherein the firstring directly bonds to the fullerene.

Another embodiment comprises a method of making a composition comprisinga mixture comprising: (i) providing at least one p-type material, (ii)providing at least one n-type material, wherein the n-type materialcomprises a fullerene derivative represented by:F*-(R)_(n)and solvates, salts, and mixtures thereof,wherein n is at least one,

F* comprises a fullerene having a surface which comprises six-memberedand five-membered rings; and

R comprises at least one optionally substituted, unsaturated orsaturated, carbocyclic or heterocyclic first ring, wherein the firstring directly bonds to the fullerene.

(iii) combining the p-type and n-type materials to form the mixture,wherein the mixture further comprises at least one solvent.

Advantages include substantially better photovoltaic efficiency,versatility with a variety of active layer systems which can be tuned toparticular applications, improved device lifetime, and improved materialand device processability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a typical conductive polymer photovoltaic (solar cell).

FIG. 2 shows improved performance with C70PCBM versus C60PCBM.

FIG. 3 provides AFM images for C60PCBM:P3HT versus C70PCBM:P3HT preparedin dichlorobenzene. Circles denote phase separation in C60PCBMsystem—these domains are absent in C70PCBM active layer.

FIG. 4 provides photovoltaic data for devices comprising indenederivatives compared to a control.

DETAILED DESCRIPTION

Introduction & Definitions

“Optionally substituted” groups refers to functional groups that may besubstituted or unsubstituted by additional functional groups. When agroup is unsubstituted by an additional group is may be referred to as agroup name, for example alkyl or aryl. When a group is substituted withadditional functional groups it may more generically be referred to assubstituted alkyl or substituted aryl.

“Carbocyclic” refers to a cyclic arrangement of carbon atoms forming aring including for example benzene or cyclohexane. Carbocyclic includesboth cycloalkyl and aryl groups. The term “cycloalkyl” refers to cyclicalkyl groups of from 3 to 20 carbon atoms having single or multiplecondensed cyclic rings which condensed rings may or may not be aromaticprovided that the point of attachment is not at an aromatic carbon atom.“Aryl” refers to an aromatic carbocyclic group of from 6 to 20 carbonatoms having a single ring (e.g., phenyl) or multiple condensed rings(e.g., naphthyl or anthryl) which condensed rings may or may not bearomatic provided that the point of attachment is at an aromatic carbonatom. Preferred aryls include phenyl, naphthyl, and the like.

“Heterocyclic” refers to a saturated, unsaturated, or heteroaromaticgroup having a single ring or multiple condensed rings, from 1 to 20carbon atoms and from 1 to 4 heteroatoms, selected from nitrogen,oxygen, sulfur, —S(O)— and —S(O)₂— within the ring. Such heterocyclicgroups can have a single ring (e.g., pyridyl or furyl) or multiplecondensed rings (e.g., indolizinyl or benzothienyl) wherein thecondensed rings may or may not be aromatic and/or contain a heteroatomprovided that the point of attachment is through an atom of the aromaticheteroaryl group. Heterocyclic groups can be for example, pyridine, orthiophene, or furan or tetrahydrofuran, pyrrole, tetrahydropyrrole,pyran, and the like. The term heterocyclic includes heteroaryl groupswhere “heteroaryl” refers to an aromatic group of from 1 to 20 carbonatoms and 1 to 4 heteroatoms selected from the group consisting ofoxygen, nitrogen, sulfur, —S(O)—, and —S(O)₂— within the ring.Heteroaryls include pyridyl, pyrrolyl, indolyl, thiophenyl, and furyl.

“Alkyl” refers to straight chain and branched alkyl groups having from 1to 20 carbon atoms, or from 1 to 15 carbon atoms, or from 1 to 10, orfrom 1 to 5, or from 1 to 3 carbon atoms. This term is exemplified bygroups such as methyl, ethyl, n-propyl, iso-propyl, n-butyl, t-butyl,n-pentyl, ethylhexyl, dodecyl, isopentyl, and the like.

“Substituted alkyl” refers to an alkyl group having from 1 to 3, andpreferably 1 to 2, substituents selected from the group consisting ofalkoxy, substituted alkoxy, acyl, acylamino, acyloxy, amino, substitutedamino, aminoacyl, aryl, substituted aryl, aryloxy, substituted aryloxy,cyano, halogen, hydroxyl, nitro, carboxyl, carboxyl esters, cycloalkyl,substituted cycloalkyl, heteroaryl, substituted heteroaryl,heterocyclic, and substituted heterocyclic.

The terms “Substituted carbocyclic,” “substituted aryl,” “substitutedcycloalkyl,” “substituted heterocyclic,” and “substituted heteroarylrefer to carbocyclic, aryl, cycloalkyl, heterocyclic, or heteroarylgroups with from 1 to 5 substituents, or optionally from 1 to 3substituents, or optionally from 1 to 2 substituents, selected from thegroup consisting of alkoxy, substituted alkoxy, acyl, acylamino,acyloxy, amino, substituted amino, aminoacyl, aryl, substituted aryl,aryloxy, substituted aryloxy, cyano, halogen, hydroxyl, nitro, carboxyl,carboxyl esters, cycloalkyl, substituted cycloalkyl, heteroaryl,substituted heteroaryl, heterocyclic, and substituted heterocyclic.

“Alkoxy” refers to the group “alkyl-O—” which includes, by way ofexample, methoxy, ethoxy, n-propyloxy, iso-propyloxy, n-butyloxy,t-butyloxy, n-pentyloxy, 1-ethylhex-1-yloxy, dodecyloxy, isopentyloxy,and the like.

“Substituted alkoxy” refers to the group “substituted alkyl-O—.”

“Alkenyl” refers to alkenyl group preferably having from 2 to 6 carbonatoms and more preferably 2 to 4 carbon atoms and having at least 1 andpreferably from 1-2 sites of alkenyl unsaturation. Such groups areexemplified by vinyl, allyl, but-3-en-1-yl, and the like.

“Substituted alkenyl” refers to alkenyl groups having from 1 to 3substituents, and preferably 1 to 2 substituents, selected from thegroup consisting of alkoxy, substituted alkoxy, acyl, acylamino,acyloxy, amino, substituted amino, aminoacyl, aryl, substituted aryl,aryloxy, substituted aryloxy, cyano, halogen, hydroxyl, nitro, carboxyl,carboxyl esters, cycloalkyl, substituted cycloalkyl, heteroaryl,substituted heteroaryl, heterocyclic, and substituted heterocyclic withthe proviso that any hydroxyl substitution is not attached to a vinyl(unsaturated) carbon atom.

“Aryloxy” refers to the group aryl-O— that includes, by way of example,phenoxy, naphthoxy, and the like.

“Alkoxy” refers to the group “alkyl-O—” which includes, by way ofexample, methoxy, ethoxy, n-propoxy, iso-propoxy, n-butoxy, t-butoxy,sec-butoxy, n-pentoxy and the like.

“Substituted alkoxy” refers to the group “substituted alkyl-O—”.

“Acyl” refers to the groups H—C(O)—, alkyl-C(O)—, substitutedalkyl-C(O)—, alkenyl-C(O)—, substituted alkenyl-C(O)—, alkynyl-C(O)—,substituted alkynyl-C(O)— cycloalkyl-C(O)—, substitutedcycloalkyl-C(O)—, aryl-C(O)—, substituted aryl-C(O)—, heteroaryl-C(O)—,substituted heteroaryl-C(O), heterocyclic-C(O)—, and substitutedheterocyclic-C(O)— wherein alkyl, substituted alkyl, alkenyl,substituted alkenyl, alkynyl, substituted alkynyl, cycloalkyl,substituted cycloalkyl, aryl, substituted aryl, heteroaryl, substitutedheteroaryl, heterocyclic and substituted heterocyclic are as definedherein.

“Acylamino” refers to the group —C(O)NRR where each R is independentlyselected from the group consisting of hydrogen, alkyl, substitutedalkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, aryl,substituted aryl, cycloalkyl, substituted cycloalkyl, heteroaryl,substituted heteroaryl, heterocyclic, substituted heterocyclic and whereeach R is joined to form together with the nitrogen atom a heterocyclicor substituted heterocyclic ring wherein alkyl, substituted alkyl,alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, cycloalkyl,substituted cycloalkyl, aryl, substituted aryl, heteroaryl, substitutedheteroaryl, heterocyclic and substituted heterocyclic are as definedherein.

“Acyloxy” refers to the groups alkyl-C(O)O—, substituted alkyl-C(O)O—,alkenyl-C(O)O—, substituted alkenyl-C(O)O—, alkynyl-C(O)O—, substitutedalkynyl-C(O)O—, aryl-C(O)O—, substituted aryl-C(O)O—, cycloalkyl-C(O)O—,substituted cycloalkyl-C(O)O—, heteroaryl-C(O)O—, substitutedheteroaryl-C(O)O—, heterocyclic-C(O)O—, and substitutedheterocyclic-C(O)O— wherein alkyl, substituted alkyl, alkenyl,substituted alkenyl, alkynyl, substituted alkynyl, cycloalkyl,substituted cycloalkyl, aryl, substituted aryl, heteroaryl, substitutedheteroaryl, heterocyclic and substituted heterocyclic are as definedherein.

“Alkynyl” refers to alkynyl group preferably having from 2 to 6 carbonatoms and more preferably 2 to 3 carbon atoms and having at least 1 andpreferably from 1-2 sites of alkynyl unsaturation.

“Substituted alkynyl” refers to alkynyl groups having from 1 to 3substituents, and preferably 1 to 2 substituents, selected from thegroup consisting of alkoxy, substituted alkoxy, acyl, acylamino,acyloxy, amino, substituted amino, aminoacyl, aryl, substituted aryl,aryloxy, substituted aryloxy, cyano, halogen, hydroxyl, nitro, carboxyl,carboxyl esters, cycloalkyl, substituted cycloalkyl, heteroaryl,substituted heteroaryl, heterocyclic, and substituted heterocyclic.

“Amino” refers to the group —NH₂.

“Substituted amino” refers to the group —NR′R″ where R′ and R″ areindependently selected from the group consisting of hydrogen, alkyl,substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substitutedalkynyl, aryl, substituted aryl, cycloalkyl, substituted cycloalkyl,heteroaryl, substituted heteroaryl, heterocyclic, substitutedheterocyclic and where R′ and R″ are joined, together with the nitrogenbound thereto to form a heterocyclic or substituted heterocylic groupprovided that R′ and R″ are both not hydrogen. When R′ is hydrogen andR″ is alkyl, the substituted amino group is sometimes referred to hereinas alkylamino. When R′ and R″ are alkyl, the substituted amino group issometimes referred to herein as dialkylamino.

“Aminoacyl” refers to the groups —NRC(O)alkyl, —NRC(O)substituted alkyl,—NRC(O)cycloalkyl, —NRC(O)substituted cycloalkyl, —NRC(O)alkenyl,—NRC(O)substituted alkenyl, —NRC(O)alkynyl, —NRC(O)substituted alkynyl,—NRC(O)aryl, —NRC(O)substituted aryl, —NRC(O)heteroaryl,—NRC(O)substituted heteroaryl, —NRC(O)heterocyclic, and—NRC(O)substituted heterocyclic where R is hydrogen or alkyl and whereinalkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl,substituted alkynyl, cycloalkyl, substituted cycloalkyl, aryl,substituted aryl, heteroaryl, substituted heteroaryl, heterocyclic andsubstituted heterocyclic are as defined herein.

“Carboxyl” refers to —COOH or salts thereof.

“Carboxyl esters” refers to the groups —C(O)O-alkyl, —C(O)O-substitutedalkyl, —C(O)Oaryl, and —C(O)O-substituted aryl wherein alkyl,substituted alkyl, aryl and substituted aryl are as defined herein.

“Cycloalkoxy” refers to —O-cycloalkyl groups.

“Substituted cycloalkoxy” refers to —O-substituted cycloalkyl groups.

“Halo” or “halogen” refers to fluoro, chloro, bromo and iodo.

“Heteroaryloxy” refers to the group —O-heteroaryl and “substitutedheteroaryloxy” refers to the group —O-substituted heteroaryl.

“Heterocyclyloxy” refers to the group —O-heterocyclic and “substitutedheterocyclyloxy” refers to the group —O-substituted heterocyclic.

“Thiol” refers to the group —SH.

“Thioalkyl” or “alkylthioether” or “thioalkoxy” refers to the group—S-alkyl.

“Substituted thioalkyl” or “substituted alkylthioether” or “substitutedthioalkoxy” refers to the group —S-substituted alkyl.

“Thiocycloalkyl” refers to the groups —S-cycloalkyl and “substitutedthiocycloalkyl” refers to the group —S-substituted cycloalkyl.

“Thioaryl” refers to the group —S-aryl and “substituted thioaryl” refersto the group —S-substituted aryl.

“Thioheteroaryl” refers to the group —S-heteroaryl and “substitutedthioheteroaryl” refers to the group —S-substituted heteroaryl.

“Thioheterocyclic” refers to the group —S-heterocyclic and “substitutedthioheterocyclic” refers to the group —S-substituted heterocyclic.

“Salts” are derived from a variety of organic and inorganic counter ionswell known in the art and include, by way of example only, sodium,potassium, calcium, magnesium, ammonium, tetraalkylammonium, and thelike; and when the molecule contains a basic functionality, salts oforganic or inorganic acids, such as hydrochloride, hydrobromide,tartrate, mesylate, acetate, maleate, oxalate and the like.

“Solvate” refers to those forms of the compounds which form, in thesolid or liquid state, a complex by coordination with solvent-molecules.Hydrates are a specific form of solvates in which the coordination takesplace with water.

“Conjugated polymer” refers to polymers comprising at least someconjugated unsaturation in the backbone.

“A polythiophene” or “polythiophene” refers to polymers comprising athiophene in the backbone including polythiophene, derivatives thereof,and copolymers and terpolymers thereof.

“Regioregular polythiophene” refers to polythiophene having high levelsof regioregularity including for example at least 80%, or at least 90%,or at least 95%, or at least 98%, or at least 99%.

It is understood that in all substituted groups defined above, polymersarrived at by defining substituents with further substituents tothemselves (e.g., substituted aryl having a substituted aryl group as asubstituent which is itself substituted with a substituted aryl group,etc.) are not intended for inclusion herein. In such cases, the maximumnumber of such substituents is three. That is to say that each of theabove definitions is constrained by a limitation that, for example,substituted aryl groups are limted to -substituted aryl-(substitutedaryl)-substituted aryl.

Similarly, it is understood that the above definitions are not intendedto include impermissible substitution patterns (e.g., methyl substitutedwith 5 fluoro groups or a hydroxyl group alpha to ethenylic oracetylenic unsaturation). Such impermissible substitution patterns arewell known to the skilled artisan.

Other terms used herein are defined as follows, unless the context makesclear otherwise.

All references cited herein are incorporated by reference in theirentirety.

Solar cells are described in for example Hoppe and Sariciftci, J. Mater.Res., Vol. 19, No. 7, July 2004, 1924-1945, which is hereby incorporatedby reference including the figures.

FIG. 1 illustrates some components of a conventional solar cell. Seealso for example Dennler et al., “Flexible Conjugated Polymer-BasedPlastic Solar Cells: From Basics to Applications,” Proceedings of theIEEE, vol. 93, no. 8, August 2005, 1429-1439, including FIGS. 4 and 5.Various architectures for the solar cell can be used, including invertedsolar cells. Important elements include the active layer, an anode, acathode, and a substrate to support the larger structure. In addition, ahole injection layer can be used, and one or more conditioning layerscan be used. The active layer can comprise a P/N composite including aP/N bulk heterojunction.

The following references describe photovoltaic materials and devices:

US Patent Publication 2006/0076050 to Williams et al., “HeteroatomicRegioregular Poly(3-Substitutedthiophenes) for Photovoltaic Cells,”(Plextronics) which is hereby incorporated by reference includingworking examples and drawings.

US Patent Publication 2006/0237695 (Plextronics), “Copolymers of SolublePoly(thiophenes) with Improved Electronic Performance,” which is herebyincorporated by reference including working examples and drawings.

U.S. Pat. No. 7,147,936 to Louwet et al.

In addition, US Patent Publication 2006/0175582 “HoleInjection/Transport Layer Compositions and Devices” describes holeinjection layer technology, (Plextronics) which is hereby incorporatedby reference including working examples and drawings.

Device Elements Other than the Active Layer

Electrodes, including anodes and cathodes, are known in the art forphotovoltaic devices. See, for example, Hoppe et al. article citedabove. Known electrode materials can be used. Transparent conductiveoxides can be used. Transparency can be adapted for a particularapplication. For example, the anode can be indium tin oxide, includingITO supported on a substrate. Substrates can be rigid or flexible.

If desired, hole injection and hole transport layers can be used. An HILlayer can be for example PEDOT:PSS as known in the art. See, forexample, Hoppe et al. article cited above.

Active Layer P-Type Material

The active layer can comprise at least one p-type material, and thefullerene derivative n-type materials can be used in combination withvarious p-type materials. The advantage of some embodiments of theinvention is that the substituents used to derivatize the fullerene canbe chosen based on the calculated LUMO level or the calculated electronaffinity. The goal in these embodiments can be to maximize thedifference between the LUMO level of the n-type with the HOMO level ofthe p-type, while still maintaining photo carrier generation within theactive layer.

The p-type material can be an organic material including a polymericmaterial, although other types of p-type material are known in the art.For example, the p-type material can comprise a conjugated polymer or aconducting polymer, comprising a polymer backbone having a series ofconjugated double bonds. It can be a homopolymer or a copolymerincluding a block copolymer or a random copolymer, or a terpolymer.Examples include polythiophene, polypyrrole, polyaniline, polyfluorene,polyphenylene, polyphenylene vinylene, and derivatives, copolymers, andmixtures thereof. The p-type material can comprise a conjugated polymersoluble or dispersible in organic solvent or water. Conjugated polymersare described in for example T. A. Skotheim, Handbook of ConductingPolymers, 3^(rd) Ed. (two vol), 2007; Meijer et al., Materials Scienceand Engineering, 32 (2001), 1-40; and Kim, Pure Appl. Chem., 74, 11,2031-2044, 2002. The p-type active material can comprise a member of afamily of similar polymers which have a common polymer backbone but aredifferent in the derivatized side groups to tailor the properties of thepolymer. For example, a polythiophene can be derivatized with alkyl sidegroups including methyl, ethyl, hexyl, dodecyl, and the like.

One embodiment comprises copolymers and block copolymers which comprise,for example, a combination of conjugated and non-conjugated polymersegments, or a combination of a first type of conjugated segment and asecond type of conjugated segment. For example, these can be representedby AB or ABA or BAB systems wherein, for example, one block such as A isa conjugated block and another block such as B is an non-conjugatedblock or an insulating block. Or alternately, each block A and B can beconjugated. The non-conjugated or insulating block can be for example anorganic polymer block, an inorganic polymer block, or a hybridorganic-inorganic polymer block including for example addition polymerblock or condensation polymer block including for example thermoplastictypes of polymers, polyolefins, polysilanes, polyesters, PET, and thelike. Block copolymers are described in, for example, U.S. Pat. No.6,602,974 to McCullough et al., and US Patent Publication No.2006/0278867 to McCullough et al. published Dec. 14, 2006, eachincorporated herein by reference in its entirety.

In particular, polythiophenes and derivatives thereof are known in theart. They can be homopolymers or copolymers, including block copolymers.They can be soluble or dispersible. They can be regioregular. Inparticular, optionally substituted-alkoxy- and optionally substitutedalkyl-substituted polythiophenes can be used. In particular,regioregular polythiophenes can be used as described in for example U.S.Pat. Nos. 6,602,974 and 6,166,172 to McCullough et al., as well asMcCullough, R. D.; Tristram-Nagle, S.; Williams, S. P.; Lowe, R. D.;Jayaraman, M. J. Am. Chem. Soc. 1993, 115, 4910, including homopolymersand block copolymers. See also Plextronics (Pittsburgh, Pa.) commercialproducts. Soluble alkyl- and alkoxy-substituted polymers and copolymerscan be used including poly(3-hexylthiophene). Other examples can befound in U.S. Pat. Nos. 5,294,372 and 5,401,537 to Kochem et al. U.S.Pat. Nos. 6,454,880 and 5,331,183 further describe active layers.

Soluble materials or well dispersed materials can be used in the stackto facilitate processing.

Additional examples of p-type materials and polythiophenes can be foundin WO 2007/011739 (Gaudiana et al.) which describes polymers havingmonomers which are, for example, substituted cyclopentadithiophenemoieties, and which is hereby incorporated by reference in its entiretyincluding formulas.

Active Layer N-Type Material

The active layer can comprise an n-type material comprising at least onefullerene structure. Fullerenes are known in the art. Fullerenes can bedescribed as spheroidal carbon compounds. For example, the fullerenesurface can present [6,6] bonding and [6,5] bonding as known in the art.The fullerene can have a surface comprising six-membered andfive-membered rings. Fullerenes can be for example C60, C70, or C84, andadditional carbon atoms can be added via derivative groups. See forexample Hirsch, A.; Brettreich, M., Fullerenes: Chemistry and Reactions,Wiley-VCH Verlag, Weinheim, 2005, which is hereby incorporated byreference including teachings for fullerene nomenclature and synthesis,derivatization, reduction reactions (Chapter 2), nucleophilic additions(Chapter 3), cycloadditions (Chapter 4), hydrogenation (Chapter 5),radical additions (Chapter 6), transition metal complex formation(Chapter 7), oxidation and reactions with electrophiles (Chapter 8),halogenation (Chapter 9), regiochemistry (Chapter 10), clustermodification (Chapter 11), heterofullerenes (Chapter 12), and higherfullerenes (Chapter 13). Methods described herein can be used tosynthesize fullerene derivatives and adducts.

In particular, the active layer can comprise at least one n-typematerial, wherein the n-type material comprises at least one derivatizedfullerene or fullerene derivative. The derivative compound can be forexample an adduct. The terms “derivatized fullerene,” “fullerenederivative” as used herein, can be used interchangeably and can be forexample fullerenes comprising, from 1 to 84, or 1 to 70, or 1 to 60,from 1 to 20, from 1 to 18, from one to ten, or from one to six, or fromone to five, or from one to three substituents each covalently bondedto, for example, one or two carbons in the spheroidal carbon compounds.The derivatized fullerene can comprise a fullerene covalently bonded by[4+2] cycloaddition to at least one derivative moiety, R.

Structures for the n-type material can be represented by:F*-(R)_(n)and solvates, salts, and mixtures thereof,wherein

n is at least one;

F is a spheroidal fullerene having a surface which comprisessix-membered and five-membered rings; and

R comprises at least one optionally substituted, unsaturated orsaturated, carbocyclic or heterocyclic first ring, wherein the firstring directly bonds to the fullerene.

Formula (I) represents an embodiment wherein C60 is bonded to n Rgroups, and the bonding is generically represented.

The first ring can be substituted. The first ring can be notsubstituted. The first ring can be an unsaturated ring. The first ringcan be a saturated ring. The first ring can be a carbocyclic ring. Thefirst ring can be a heterocyclic ring.

The first ring can be an optionally substituted four-membered,five-membered, or six-membered ring. It can in particular be anoptionally substituted five-membered ring.

The R group can further comprise a second ring which is bonded to orfused with the first ring. The second ring can be optionallysubstituted. The second ring can be for example an aryl group which isfused to the first ring.

The first ring directly bonds to the fullerene. For example, the R groupcan covalently bond to the fullerene by a [4+2] cycloaddition. The Rgroup can be covalently bonded to the fullerene by one or two covalentbonds, including two covalent bonds, including by two carbon-carbonbonds. The R group can be bonded to the fullerene surface by a covalentbond to one atom in the R group. Alternatively the R group can be bondedto the fullerene surface by covalent bonds to two atoms in the R group.The two atoms in the R group bonded to the fullerene can be adjacent toeach other, or can be separated by from each other by 1 to 3 other atomsin the R group. The R group can be covalently bonded to the fullerene bytwo carbon-carbon bonds at a fullerene [6,6] position.

The fullerene can comprise only carbon. The fullerene can comprise atleast one derivative group bonded to the fullerene besides R.

For example, fullerenes can be derivatized with electron withdrawinggroups or electron releasing groups. Electron withdrawing groups andelectron releasing groups are known in the art and can be found inAdvanced Organic Chemistry, 5th Ed, by Smith, March, 2001.

The electron withdrawing group can be attached directly to the fullerenecage or via methano-bridges similar to the PCBM structure.

The electron donating group can be attached directly to the fullerenecage or via methano-bridges similar to the PCBM structure.

Fullerenes can be derivatized to improve their absorption in the visiblerange, relative to C60-PCBM. Improved absorption in the visible rangemay increase or improve the photocurrent of a photovoltaic devicecomprising the derivatized fullerene.

In one embodiment, F* is selected from C60, C70 and C84, andcombinations thereof.

In one embodiment, R is selected from optionally substituted aryl andoptionally substituted heteroaryl.

In one embodiment, R is selected from optionally substituted indene,optionally substituted naphthyl, optionally substituted phenyl,optionally substituted pyridinyl, optionally substituted quinolinyl,optionally substituted cyclohexyl, and optionally substitutedcyclopentyl.

In one embodiment R is selected from indene, naphthyl, phenyl,pyridinyl, quinolinyl, cyclohexyl and cyclopentyl.

The value n can be an integer. In one embodiment, n can be from 1 to 84,or from 1 to 70, or from 1 to 60, or from 1 to 30, or from 1 to 10. Inone embodiment n is from 1 to 6. In one embodiment n is from 1 to 3.

In one embodiment n is 1. In one embodiment n is 2. In one embodiment nis 3.

In one embodiment, the first ring is optionally substituted with atleast one substituent selected from the group consisting of hydroxy,acyl, acylamino, acyloxy, alkyl, substituted alkyl, alkoxy, substitutedalkoxy, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl,amino, substituted amino, aminoacyl, aryl, substituted aryl, aryloxy,substituted aryloxy, cycloalkoxy, substituted cycloalkoxy, carboxyl,carboxyl esters, cyano, thiol, thioalkyl, substituted thioalkyl,thioaryl, substituted thioaryl, thioheteroaryl, substitutedthioheteroaryl, thiocycloalkyl, substituted thiocycloalkyl,thioheterocyclic, substituted thioheterocyclic, cycloalkyl, substitutedcycloalkyl, halo, nitro, heteroaryl, substituted heteroaryl,heterocyclic, substituted heterocyclic, heteroaryloxy, substitutedheteroaryloxy, heterocyclyloxy, or substituted heterocyclyloxy, orcombination thereof.

In one embodiment n is 1 and R is indene. In one embodiment n is 2 and Ris indene. In one embodiment n is 3 and R is indene. In one embodiment nis 4 and R is indene. In one embodiment n is 5 and R is indene. In oneembodiment n is 6 and R is indene.

In one embodiment, R can be covalently bonded to the fullerene by [4+2]cycloaddition, alternatively called a [4+2] cycloadduct. Reactionsincluding [4+2] cycloaddition reactions and Diels-Alder reactions aregenerally known in the art. A dienophile double bond can react with adiene to produce a six membered ring. See for example Advanced OrganicChemistry, Reactions, Mechanisms, and Structure, 2^(nd) Ed., J. March,1977, including chapters on addition to carbon-carbon multiple bonds(e.g., Chapter 15). See also, Belik et al., Angew. Chem., Int. Ed. Engl.1993, 32, 1, 78-80 (showing reaction of C60 with a C8 o-quinodimethanecompound to form a C68 compound comprising the fullerene and thederivative moiety); and Puplovskis et al., Tetrahedron Letters, 38, 2,285-288, 1997, 285-288 (showing reaction of C60 with C9 indene to form aC69 compound comprising the fullerene and the derivative moiety). Thecycloaddition reaction can result in reaction at the [6,6] fullerenedouble bonds rather than [6,5] double bonds. Cycloaddition reactions aredescribed in detail in Chapter 4, pages 101-183, of the Hirsch,Brettreich text, Fullerenes, Chemistry and Reactions, 2005.

One example of a fullerene derivative is an indene derivative. Inaddition, indene itself can be derivatized. Fullerene can be derivatizedby methods described in for example Belik et al., Angew. Chem. Int. Ed.Engl., 1993, 32, No. 1, pages 78-80, which is hereby incorporated byreference. This paper describes addition to electron poor superalkene,C60, which can add radicals such as o-quinodimethane. It can be preparedin situ containing different functional groups and form very reactivedienes that can form [4+2] cycloadducts even with the least reactivedienophiles. This method provides good selectivity and stability.

The fullerene can comprise at least two derivative moieties, R, to formbis-adducts or at least three derivative moieties, R, to formtris-adducts. These substituents can be added to the fullerene by [4+2]cycloaddition. For example, Belik et al. show in Scheme 1, formula 3, afullerene compound comprising two derivative moieties. In addition, twofullerenes can be covalently linked by one derivative moiety as shown inScheme 2 of Belik et al.

While the various embodiments are not limited by theory, it is believedthat the derivatization may disrupt the conjugation of the fullerenecage. Disrupting the conjugation effects the ionization potential andelectron affinity of the derivatized fullerene.

In one embodiment, the active layer can comprise at least onepolythiophene and at least one fullerene derivative comprising anelectron withdrawing group.

Device Fabrication

Devices using the presently claimed inventions can be made using forexample ITO as an anode material on a substrate. Other anode materialscan include for example metals, such as Au, carbon nanotubes, single ormultiwalled, and other transparent conducting oxides. The resistivity ofthe anode can be maintained below for example 15 Ω/sq or less, 25 orless, 50 or less, or 100 or less, or 200 or less, or 250 or less. Thesubstrate can be for example glass, plastics (PTFE, polysiloxanes,thermoplastics, PET, PEN and the like), metals (Al, Au, Ag), metalfoils, metal oxides, (TiOx, ZnOx) and semiconductors, such as Si. TheITO on the substrate can be cleaned using techniques known in the artprior to device layer deposition. An optional hole injection layer (HIL)can be added using for example spin casting, ink jetting, doctorblading, spray casting, dip coating, vapor depositing, or any otherknown deposition method. The HIL can be for example PEDOT, PEDOT/PSS orTBD, or NPB, or Plexcore HIL (Plextronics, Pittsburgh, Pa.).

The thickness of the HIL layer can be for example from about 10 nm toabout 300 nm thick, or from 30 nm to 60 nm, 60 nm to 100 nm, or 100 nmto 200 nm. The film then can be optionally dried/annealed at 110 to 200°C. for 1 min to an hour, optionally in an inert atmosphere.

The active layer can be formulated from a mixture of n-type and p-typematerials. The n- and p-type materials can be mixed in a ratio of forexample from about 0.1 to 4.0 (p-type) to about 1 (n-type) based on aweight, or from about 1.1 to about 3.0 (p-type) to about 1 (n-type) orfrom about 1.1 to about 1.5 (p-type) to about 1 (n-type). The amount ofeach type of material or the ratio between the two types of componentscan be varied for the particular application.

The n- and p-type materials can be mixed in a solvent or in a solventblend at for example from about 0.01 to about 0.1% volume solids. Thesolvents useful for the presently claimed inventions can include, forexample, halogenated benzenes, alkyl benzenes, halogenated methane, andthiophenes derivatives, and the like. More specifically, solvent can befor example cholobenzene, dichlorobenzene, xylenes, toluene, chloroform,3-methylthiophene, 3-propylthiphene, 3-hexylthiphene, and mixturesthereof. At least two solvents can be used.

Particularly useful solvent systems can be used as described inco-pending U.S. patent application entitled “Solvent System forConjugated Polymers,” Ser. No. 60/915,632 filed on May 2, 2007, toSheina et al., which is hereby incorporated by reference in itsentirety.

The active layer can be then deposited by spin casting, ink jetting,doctor blading, spray casting, dip coating, vapor depositing, or anyother known deposition method, on top of the HIL film. The film is thenoptionally annealed at for example about 40 to about 250° C., or fromabout 150 to 180° C., for about 10 min to an hour in an inertatmosphere.

Next, a cathode layer can be added to the device, generally using forexample thermal evaporation of one or more metals. For example, a 1 to15 nm Ca layer is thermally evaporated onto the active layer through ashadow mask, followed by deposition of a 10 to 300 nm Al layer.

In some embodiments and optional interlayer may be included between theactive layer and the cathode, and/or between the HTL and the activelayer. This interlayer can be for example from 0.5 nm to about 100 nm,or from about 1 to 3 nm, thick. The interlayer can comprise an electronconditioning, a hole blocking, or an extraction material such as LiF,BCP, bathocuprine, fullerenes or fullerene derivatives, such as C60 andother fullerenes and fullerene derivatives discussed herein.

The devices can be then encapsulated using a glass cover slip sealedwith a curable glue, or in other epoxy or plastic coatings. Cavity glasswith a getter/dessicant may also be used.

In addition, the active layer can comprise additional ingredientsincluding for example surfactants, dispersants, and oxygen and waterscavengers.

The active layer can comprise multiple layers or be multi-layered.

The active layer composition can comprise a mixture in the form of afilm.

Active Layer Morphology

The active layer can be a p-n composite and for example can form aheterojunction including a bulk heterojunction. See for examplediscussion of nanoscale phase separation in bulk heterojunctions inDennler et al., “Flexible Conjugated Polymer-Based Plastic Solar Cells:From Basics to Applications,” Proceedings of the IEEE, vol. 93, no. 8,August 2005, 1429-1439. Conditions and materials can be selected toprovide for good film formation, low roughness (e.g., 1 nm RMS), anddiscrete, observable, phase separation characteristics can be achieved.The present invention can have phase separated domains on a scale of aabout 5 to 50 nm as measured by AFM. AFM analysis can be used to measuresurface roughness and phase behavior. In general, phase separateddomains are not desirable so that both donor and acceptor are uniformlyand continuously distributed in the active layer.

Device Performance

Known solar cell parameters can be measured including for example J_(SC)(mA/cm²) and Voc (V) and fill factor (FF) and power conversionefficiency (%, PCE) by methods known in the art. See for example Hoppearticle cited above and references cited therein.

For example, the efficiency can be at least about 2%, or at least about3%, or at least about 3.5%, or at least about 4%, or at least about4.5%, or at least about 5.0%, or at least about 5.5%, or at least about6.0%, or at least about 7.0%, or at least about 8.0%, or at least about9.0%, or at least about 10.0% at 1 sun (AM1.5G, 100 mW/cm²). Anefficiency range can be for example about 2% to about 15%, or about 2%to about 10%, or about 2% to about 7%. These efficiencies can beverified by NREL.

The fill factor, for example, can be at least about 0.60, or at leastabout 0.63, or at least about 0.67, at least about 0.7, at least about0.75, or at least about 0.8, at least about 0.85.

The Voc (V), for example, can be at least about 0.56, or at least about0.63, or at least about 0.82, at least about 0.9, at least about 1.0, atleast about 1.2, at least about 1.4, at least about 1.5.

The Jsc (mA/cm²), for example, can be at least about 8.92, or at leastabout 9.20, or at least about 9.48, or at least about 10, or at leastabout 11, or at least about 12, or at least about 13, or at least about14, or at least about 15.

The device can demonstrate an increase of efficiency of at least 5%, orat least 15%, compared to a substantially analogous device comprising anactive layer of P3HT-PCBM.

FIG. 2 illustrates improved performance, wherein efficiency is raised to3% rather than 2% for control devices (at least 20% improvement). Theimprovement in efficiency is consistent with the significant extensionto higher wavelengths of the absorption spectrum for C70PCBM and theresulting blend with p-type material compared to that of C60PCBM-basedfilms. Additional comparative data are provided in FIG. 3 showing betterfilm morphology.

Oriel Solar Simulators can be used to determine PV properties includingfor example FF, Jsc, Voc, and efficiencies. The simulator can becalibrated by methods known in the art including for example calibrationwith a KG5-Si reference cell.

LITERATURE

The following references can be also used as needed to practice thevarious embodiments described herein and are incorporated herein byreference:

REFERENCES CITED

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Embodiment 1. A photovoltaic device comprising: a first electrode,

a second electrode, an least one active layer disposed between the firstand second electrodes, wherein the active layer comprises at least onepolythiophene and at least one fullerene derivative comprising anelectron withdrawing group.

2. The device according to embodiment 1, wherein the fullerenederivative is a C60 fullerene.

3. The device according to embodiment 1, wherein the fullerenederivative is a C70 fullerene.

4. The device according to embodiment 1, wherein the fullerenederivative is a C84 fullerene.

5. The device according to embodiment 1, wherein the fullerenederivative comprises C6006, C60(C₉H₈), C60Br24b, C60Cl(CH2ClChC12)

6. The device according to embodiment 1, wherein the polythiophene is aregioregular polythiophene.

7. The device according to embodiment 1, wherein the polythiophene is acopolymer.

8. The device according to embodiment 1, further comprising an HIL orHTL layer.

9. The device according to embodiment 1, wherein the polythiophene is asoluble regioregular polythiophene.

10. The device according to embodiment 1, wherein the electronwithdrawing group also generates absorption in the visible spectrum.

11. A photovoltaic device comprising:

a first electrode,

a second electrode,

an least one active layer disposed between the first and secondelectrodes, wherein the active layer comprises at least onepolythiophene and at least one fullerene derivative, wherein thefullerene derivative is a C70 or C84 fullerene.

12. A photovoltaic device comprising:

a first electrode,

a second electrode, an least one active layer disposed between the firstand second electrodes, wherein the active layer comprises at least onepolythiophene and at least one fullerene derivative, wherein thefullerene derivative is a C70 fullerene.

13. A photovoltaic device comprising:

a first electrode,

a second electrode,

an least one active layer disposed between the first and secondelectrodes, wherein the active layer comprises at least onepolythiophene and at least one fullerene derivative, wherein thefullerene derivative is a C84 fullerene.

14. A photovoltaic device comprising:

a first electrode,

a second electrode,

an least one active layer disposed between the first and secondelectrodes, wherein the active layer comprises at least onepolythiophene and underivatized C70 fullerene.

15. A photovoltaic device comprising:

a first electrode,

a second electrode,

an least one active layer disposed between the first and secondelectrodes, wherein the active layer comprises at least onepolythiophene and underivatized C84 fullerene.

16. A photovoltaic device comprising:

a first electrode,

a second electrode,

an least one active layer disposed between the first and secondelectrodes, wherein the active layer comprises at least onepolythiophene and at least one fullerene derivative comprisingo-quinodimethane derivative group.

WORKING EXAMPLES

Various claimed embodiments are described further with use ofnon-limiting working examples.

Example 1 Synthesis of C₆₀-Indene Adducts

C₆₀ indene adducts have been synthesized using the description inreference (Puplovskis, et al., “New Route for [60]FullereneFunctionalization in [4+2] Cycloaddition Reaction Using Indene.”Tetrahedron Lett. 1997, 38, 285-288) as starting point. C₆₀ wasdissolved in o-dichlorobenzene at concentrations of approximately 6 mgmL⁻¹. Indene was added at 12-fold molar excess relative to C₆₀ and theresulting mixture was refluxed overnight. Most of the solvent wasevaporated under reduced pressure and precipitation occurred afteradding ethanol. The resulting solid was dried, re-dissolved in tolueneand then analyzed by means of high-pressure liquid chromatography usinga Cosmosil Buckyprep analytical column (250×4.6 mm, Nacalai Tesque,Inc.) mounted on an Agilent 1100 series instrument equipped with avariable wavelength detector operated at 330 nm. Toluene at a flow rateof 1 ml min⁻¹ was used for elution. Peaks at an elution time ofapproximately 5.4 min and a group of peak around 4 min were attributedto C₆₀-indene mono- and bis-adducts, respectively. The presence ofseveral peaks with very close elution times around 4 min is consistentwith the presence of several isomers. Elution times shorter than that ofunfunctionalized C60 (about 8.1 min) have been observed previously withother fullerene derivatives such as C₆₀PCBM. Multiple additions werefound to lead to a further decrease of elution times. Apentabromobenzyl-functionalized silica phase was used for purificationby means of flash chromatography. Pure toluene and toluene/cyclo-hexanemixtures were used for purification. HPLC analysis of the collectedfractions showed purities 98.5% for the C₆₀-indene monoadduct andapproximately 95% for the mixture of different bis-adducts.

Example 2 Synthesis of C₇₀-Indene Monoadduct

C70-indene mono-adduct was synthesized following the procedure developedfor the C60-indene adducts. C₇₀ was dissolved in o-dichlorobenzene.After addition of indene in a 12-fold molar excess, reflux wasmaintained for 8 h. After reduction of the volume under reduced pressureand addition of ethanol, solid was recovered, dried and re-dissolved intoluene. HPLC analysis using the same procedure as described aboveshowed the presence of mainly mono-adduct, probably due to the reactiontime, reduced in comparison to the C₆₀-adduct synthesis. Purificationusing flash chromatography led to the isolation of C₇₀-monoadduct at apurity of 98.6%. The corresponding HPLC chromatogram is given below. Twomajor isomers representing different addition sites on the C₇₀ cage havebeen identified.

Example 3 Preparation of Polythiophene

Plexcore P3HT was prepared as described in Loewe, et al. Adv. Mater.1999, II, 250-253 using 2,5-dibromo-3-hexylthiophene in place of2,5-dibromo-dodecylthiophene, and using 0.0028 eq. of Ni(dppp)Cl₂instead of 0.01 eq. The molecular weight as measured by GPC usingchloroform as eluant was 69,000, 1.35 PDI.

Example 4 Fabrication of Solar Cell Device Using C60 Indene Adducts

Photovoltaic devices were prepared comprising (i) patterned indium tinoxide (ITO, anode, 60 Ω/square) on glass substrate purchased from ThinFilm Devices (located in Anaheim, Calif.), (ii) a thin layer of HIL (30nm thick) comprising PEDOT/PSS (AI4083) purchased from HC Stark), (iii)a 100 nm active layer comprising Plexcore P3HT (prepared as described inExample 3) blended with the n-type, which is either methanofullerence[6,6]-phenyl C61-butyric acid methyl ester (PCBM) (purchased fromNano-C, located in Westwood, Mass.), C₆₀-Indene mono adduct, orC₆₀-indene bis-adduct, (the fullerene adducts prepared as described inabove examples), and (iv) a Ca/Al bilayer cathode.

The patterned ITO glass substrates were cleaned with detergent, hotwater and organic solvents (acetone and alcohol) in an ultrasonic bathand treated with ozone plasma immediately prior to device layerdeposition. The HIL solution (Baytron AI 4083) was then spin coated onthe patterned ITO glass substrate to achieve a thickness of 30 nm. Thefilm was dried at 150° C. for 30 mins in a nitrogen atmosphere. Theactive layer was formulated to a 1.2:1 weight ratio P3HT:n-type blend ino-dichlorobenzene (formulation was made to 0.024% volume solids) and wasthen spun on the top of the HIL film with no damage to the HIL (verifiedby AFM).

The film was then annealed at 175° C. for 30 mins in a glove box. Next,a 5 nm Ca layer was thermally evaporated onto the active layer through ashadow mask, followed by deposition of a 150 nm Al layer. The deviceswere then encapsulated via a glass cover slip (blanket) encapsulationsealed with EPO-TEK OG112-4 UV curable glue. The encapsulated device wascured under UV irradiation (80 mW/cm²) for 4 minutes and tested asfollows.

The photovoltaic characteristics of devices under white light exposure(Air Mass 1.5 Global Filter) were measured using a system equipped witha Keithley 2400 source meter and an Oriel 300W Solar Simulator based ona Xe lamp with output intensity of 100 mW/cm² (AM1.5G). The lightintensity was set using an NREL-certified S1-KG5 silicon photodiode.

The Jsc, Voc and efficiency measured for each device are shown in thetable below compared to a control device which was made as describedabove using PCBM as the n-type material. The data are furtherillustrated in FIG. 4.

TABLE FIG. 4 Jsc (mA/cm²) Voc (V) FF Efficiency (%) Control --□-- 8.920.56 0.66 3.3 Mono-indene --∘-- 9.20 0.63 0.67 3.9 adduct Bis-indene--Δ-- 9.48 0.82 0.63 4.9 adduct Bis-indene 9.43 0.84 0.64 5.1 adduct

What is claimed is:
 1. A composition comprising at least one n-typematerial, wherein the n-type material comprises a fullerene derivativerepresented by:F*-(R)_(n) and solvates, salts, and mixtures thereof, wherein n is 1 or2, F* comprises a fullerene having a surface which comprisessix-membered and five-membered rings; and R is optionally substitutedo-quinodimethane which is covalently bonded to the fullerene by twobonds; and wherein the composition further comprises at least one p-typematerial mixed with said n-type material, wherein said p-type materialis poly(3-hexylthiophene).
 2. The composition of claim 1, wherein R isunsubstituted o-quinodimethane.
 3. The composition of claim 1, wherein Ris substituted with at least one substituent selected from the groupconsisting of hydroxy, acyl, acylamino, acyloxy, alkyl, substitutedalkyl, alkoxy, substituted alkoxy, alkenyl, substituted alkenyl,alkynyl, substituted alkynyl, amino, substituted amino, aminoacyl, aryl,substituted aryl, aryloxy, substituted aryloxy, cycloalkoxy, substitutedcycloalkoxy, carboxyl, carboxyl esters, cyano, thiol, thioalkyl,substituted thioalkyl, thioaryl, substituted thioaryl, thioheteroaryl,substituted thioheteroaryl, thiocycloalkyl, substituted thiocycloalkyl,thioheterocyclic, substituted thioheterocyclic, cycloalkyl, substitutedcycloalkyl, halo, nitro, heteroaryl, substituted heteroaryl,heterocyclic, substituted heterocyclic, heteroaryloxy, substitutedheteroaryloxy, heterocyclyloxy, or substituted heterocyclyloxy, andcombination thereof.
 4. The composition of claim 1, wherein R iscovalently bonded to the fullerene by [4+2]cycloaddition.
 5. Thecomposition of claim 1, wherein R is covalently bonded to at least onefullerene [6,6] bonding site by two carbon-carbon bonds.
 6. Thecomposition of claim 1, wherein the fullerene comprises a C60, C70, orC84 fullerene, or a combination thereof.
 7. The composition of claim 1,wherein the fullerene derivative comprises at least one derivative groupbonded to the fullerene besides R.
 8. The composition of claim 1,wherein the composition further comprises at least one solvent.
 9. Thecomposition of claim 1, wherein n is
 2. 10. The composition of claim 1,wherein n is
 1. 11. A photovoltaic device comprising at least one anode,at least one cathode, and at least one active layer, wherein the activelayer comprises the composition of claim
 1. 12. The photovoltaic deviceof claim 11, wherein the power conversion efficiency of the device is atleast 4%.
 13. The photovoltaic device of claim 11, wherein the devicedemonstrates an increase of efficiency of at least 15% compared to asubstantially analogous device comprising an active layer of P3HT-PCBM.14. The photovoltaic device of claim 11, further comprising at least onehole injection layer or hole transport layer which comprisespolythiophene.